The lining of blood vessel is capable of secreting a substance that is capable of causing relaxation of smooth muscle that underlies the lining of blood vessels. This relaxation is important to maintain normal blood vessel tone and deficiency in the secretion of the dilating substance called endothelium-derived relaxant factor, can lead to increase in blood vessel tone which may translate to high blood pressure and spasm of the coronary arteries. L-arginine, and amino acid which is part of the normal diet, is capable of increasing the production of endothelium- derived relaxant factor. In this study we investigated the effect of L- arginine given into the heart arteries to investigate whether this would improve the reduced relaxation blood vessels of the heart. In patients with reduced secretion of endothelium-derived relaxant factor, infusion of arginine helped cause further relaxation.